The present invention relates to a liquid crystal display and a manufacturing method thereof, and in particular, to homeotropic and hybrid types of liquid crystal devices and manufacturing methods thereof.
FIG. 5 shows the structure of a electrically controlled birefringence liquid crystal display cell. There are two types of liquid crystal display cells, that is, reflection and transmission; the transmission type will be described herein below as a typical one.
The configuration in FIG. 5 comprises a liquid crystal cell containing liquid crystal molecules 1 whose longitudinal axis is homeotropic-aligned, that is, aligned in a direction substantially perpendicular to the electrode surface.
In FIG. 5, a liquid crystal cell 2 includes two transparent glass substrates 3 and 4 oppositely arranged at a predetermined distance, transparent electrodes 5 and 6 formed on the opposite surfaces of the transparent glass substrates 3 and 4, respectively, and liquid crystals 1 sandwiched between the transparent electrodes.
There are placed, over and under the cell 2, polarizers 7 and 8 whose polarization directions are orthogonal to each other.
Incident light 9, when passing through the polarizer 8, is linearly polarized and then goes into the liquid crystal cell. In the proximity of the interface between the two substrates 3 and 4, the liquid crystal molecules are slightly tilted (pretilted) toward an appropriate direction through an alignment process in order to tilt the liquid crystal molecules uniformly in the plane.
If the slight tilt is ignored, the linearly-polarized incident light passes through the liquid crystal molecules 1 as it is, but it cannot pass through the polarizer 7 placed perpendicularly to the polarization axis of the polarizer 8, and thus the resulting display is in a dark state.
When a voltage higher than a predetermined threshold voltage is applied between the transparent electrodes 5 and 6, the alignment of the liquid crystal molecules 1 is tilted by the electric field to form a predetermined angle.
Accordingly, after going into the liquid crystal cell 2, the linearly-polarized incident light is doubly refracted into two components orthogonal to each other and then the polarized light having a component parallel to the polarization axis passes through the polarizer 7 to bring the display to a bright state.
At this time, a high contrast is achieved by placing both the two polarizers 7 and 8 at an angle of 45 degrees with respect to the tilt direction (pretilt direction) of the liquid crystal molecules.
Next, the structure of a hybrid liquid crystal cell will be described below.
The configuration in FIG. 6 exhibits a homeotropic alignment near a substrate 11 wherein the longitudinal axis of a liquid crystal molecule 10 is aligned in a direction substantially perpendicular to the surface of a transparent electrode 13 and a homogeneous alignment near another substrate 12 wherein the longitudinal axis of a liquid crystal molecule 10 is aligned in a direction substantially parallel to a transparent electrode 14.
In FIG. 6, a liquid crystal cell 15 includes two transparent glass substrates 11 and 12 oppositely arranged at a predetermined distance, transparent electrodes 13 and 14 formed on the opposite surfaces of the transparent glass substrates 11 and 12, respectively, and liquid crystals 10 sandwiched between the transparent electrodes 13 and 14.
There are placed, over and under the liquid crystal cell 15, polarizers 16 and 17 whose polarization directions are orthogonal to each other and the polarizers are placed in such a manner that both the polarization axes of these plates are at an angle of 45 degrees with respect to the homogeneous alignment near the glass substrate 12.
Incident light 18, when passing through the polarizer 16, is linearly polarized and then goes into the liquid crystals 10. The linearly-polarized incident light, which may be slightly subject to birefringence, passes through the liquid crystal molecules substantially as it is, but it can hardly pass through the polarizer 17 placed perpendicularly to the polarization axis of the polarizer 16, and thus the resulting display is in a dark state.
When a voltage higher than a predetermined voltage is applied between the transparent electrodes 13 and 14, the alignment of the liquid crystal molecules 10 is tilted by the electric field to form a predetermined angle.
Therefore, after going into the liquid crystal cell 15, the linearly-polarized incident light is doubly refracted into two components orthogonal to each other and then the polarized light having a component parallel to the polarization axis of the polarizer 17 passes through the polarizer 17 to bring the display to a bright state.
However, as shown in FIG. 7, when a homeotropic alignment is achieved over a portion of the substrate 20 which has apixel electrode 19, that is, liquid crystal molecules 21 are aligned perpendicularly to the substrate 20, the liquid crystal molecules over the edge of the pixel electrode 19 are aligned parallel or perpendicularly to the edge of the pixel electrode 19.
Thus, while in a black display state (while no electric field is applied), the incident light around the pixel is polarized, causing some leakage of the light 23. This could have reduced the resulting contrast considerably.
In particular, for an element of reflection type, because the light passes through the liquid crystal layer twice, an outstanding leakage of light occurs around the pixel. Similarly, a remarkable leakage of light may occur when a kind of material with a large anisotropy in refractive index is used.
It is an object of the present invention to provide a liquid crystal device which causes no leakage of light while in a black display state with a resulting high contrast and to provide a manufacturing method thereof.
The first liquid crystal device of the present invention is a liquid crystal device comprising a first substrate which has at least rectangular or square electrodes divided into microstructures, a second substrate which has a transparent electrode, and a liquid crystal layer which is filled between said two substrates and has a negative anisotropy in dielectric constant wherein a homeotropic alignment is accomplished over said first substrate, characterized in that said alignment over said first substrate is accomplished at an angle of 40 to 50 degrees with respect to any of the edges of an electrode on said first substrate and a homeotropic alignment is accomplished over said second substrate. In an embodiment of the present invention, it is preferable that the angle formed through an alignment process is as nearly 45 degrees as possible.
The second liquid crystal device of the present invention is a liquid crystal device comprising a first substrate which has at least rectangular or square electrodes divided into microstructures, a second substrate which has a transparent electrode, and a liquid crystal layer which is filled between said two substrates and has a negative anisotropy in dielectric constant wherein a homeotropic alignment is accomplished over said first substrate, characterized in that a homogeneous alignment over said second substrate is accomplished at an angle of 40 to 50 degrees with respect to any of the edges of an electrode on said first substrate. In another embodiment of the present invention, it is preferable that the angle formed through an alignment process for the second substrate is as nearly 45 degrees as possible with respect to the edge of a pixel on the first substrate.
The third liquid crystal device of the present invention is a reflection type liquid crystal device comprising a first substrate which has at least rectangular or square reflecting electrodes divided into microstructures, a second substrate which has a transparent electrode, and a liquid crystal layer which is filled between said first and second substrates and has a negative anisotropy in dielectric constant and an anisotropy in refractive index An between 0.07 and 0.15 wherein a homeotropic alignment is accomplished over said first substrate, characterized in that said alignment over said first substrate is accomplished at an angle of 40 to 50 degrees with respect to any of the edges of an electrode on said first substrate and a homeotropic alignment is accomplished over said second substrate. In still another embodiment of the present invention, it is also preferable that the angle formed through an alignment process is as nearly 45 degrees as possible.
For any of the embodiments, such an alignment process may be accomplished by rubbing, irradiation of polarized ultraviolet light, irradiation of non-polarized ultraviolet light, or diagonal evaporation.